Stored signal enhancement electron discharge device



July 5, 1966 s. JENSEN ETAL 3,259,791

STORED SIGNAL ENHANCEMENT ELECTRON DISCHARGE DEVICE Filed May 51, 1962 2 Sheets-Sheet 1 S IN N E wm V mm Mu M50120 P5210350 S OF SmEmmE It; wofimnm m $365 6 43:35. A I v1 0 u 3 m mm mu w m EEZMEK .PZ OL MQOIP O P5210550 Q mw Y O E nm N e.| Don E S V N T l A w m mm y 5, 1956 A. s. JENSEN ETAL 3,259,791

STORED SIGNAL ENHANGEMENTELECTRON DISCHARGE DEVICE Filed May 31, 1962 2 Sheets-Sheet 2 DEVELOPMENT CATHODE FIRST POTENTIAL CROSSOVER I l I Y'II POTENTIAL OF STORAGE SURFACE WITH RESPECT TO DEVELOPMENT CATHODE Fig. 3 2

STORAGE TARGET LAYER r a 4 DEVELOPMENT R g CATHODE C READING BEAM 7 l A R G E T BLANKING ELECTRODE SWITCH READING BEAM LI CATHODE READI BEAM PLATE RESISTANCE DARK cuRRENT DEVELOPMENT READING HIGH LIGHT cuRRENT POTENTI L POTENTIAL ARCTAN I/R DARK READING v OUTPUT POTENTIAL OF sToRAGE SURFACE WITH RESPECT TO DEVELOPMENT CATHODE DARK cuRRENT \/READING CHARACTERISTIC H9 5 HIeH LIGHT READING OUTPUT POTENTIAL NET CURRENT TO STORAGE SURFACE United States Patent STORED SIGNAL ENHANCEMENT ELECTRON DISCHARGE DEVICE Arthur S. Jensen and Melvin P. Siedband, Baltimore, Md., assiguors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed May 31, 1962, Ser. No. 198,932 8 Claims. (Cl. 315-12) This invention relates to electron discharge devices and, more particularly, to storage and imaging devices which utilize a target member which is excitable to provide a distributed charge pattern thereon.

Several types of devices, including camera tubes such as the vidicon, iconoscope, and image orthicon, and the several varieties of storage tubes, for example, image display storage tubes, store information as a charge pattern. This charge pattern in turn gives rise to a corresponding potential field pattern which acts to control or modulate an electron reading beam. The modulation of the reading current is proportional to the strength of the stored potential pattern. To maintain independence, each resolvable elemental target area must have a capacitance to ground greater than the inter-element capacitance.

Irrespective of the manner in which the charge pattern is established on the target member of a discharge device, it is often desirable to enhance or develop a low contrast charge pattern to one in which the resolvable elements are more fully separated in their relative potential values.

It is, therefore, an object of this invention to provide an improved electron discharge device.

A further object is to provide an improved electron discharge device having target means capable of possessing a potential charge pattern.

Another object is to provide means and method for enhancing the information possessed by the target member of an electron discharge device.

A still further object is to provide means for the differential rate of change in the charge on resolvable target areas of an electron discharge device.

Another object is to provide a method of increasing the charge difference of elemental areas of a target capable of possessing an information pattern in the form of a charge pattern.

A further object is to provide a pickup tube capable of halftone reproduction and greater sensitivity.

A still further object is to provide a storage tube capable of halftone reproduction and increased writing speed.

Basically, the present invention provides that an electron discharge device is provided with a target structure which is capable of providing an information pattern thereon in the form of electrical charges on elemental areas of the target. The electron discharge device also includes an enhancing electron gun which uniformly either floods or scans the target structure with electrons. The energy at which the electrons from this gun strike the target areas is chosen so that secondary emission from more positively charged areas is higher than the secondary emission from areas less positively charged. That is, the enhancing gun is operated at a potential with respect to the target which corresponds to a positive rate of change of secondary emission (with respect to voltage) from the target. Thus, if the electrons from the enhancing gun are incident upon the target with uniform current density, the net effect resulting from secondary emission will be that the potential difference between two initially charged areas will increase. Thus, it may be seen that an information pattern which is initially very weak may be enhanced or improved to provide an information pattern of greater contrast and halftone reproduction. Such a pattern is one which may be more readily and rapidly scanned by the reading beam to produce an enhanced output. In a pickup or camera tube, this results in substantially increased sensitivity since the weak charge pattern corresponding to a weaker incoming radiation signal can be enhanced until it is detectable. In a storage tube this enhancement enables a greater writing speed to be used.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

FIGURE 1 is an elevational view, in section, of an electron discharge device embodying the present invention;

FIG. 2 is a typical secondary emission curve for materials such as those which are employed as target materials in the device of FIG. 1; and,

FIGS. 3 through 5 are supplemental figures useful in the explanation of the operation of the device of FIG. 1.

Referring in detail to FIG. 1, an evacuated vacuumtight envelope 10 of suitable material such as glass, comprises an enlarged tubular portion 11 and a smaller tubular portion 12. The enlarged portion 11 may be closed off by means of a faceplate 13, which is of a suitably Wideband transmitting material such as sapphire, calcium fluoride, or barium fluoride, and which may be integrally formed with the envelope 10. The portion 12 is sealed off and may be provided with a cap member 14, which is of a suitable material such as plastic, in a manner well known in the art.

A suitable electron beam producing source 15 is disposed near one end of the enevelope 10 within the portion 12. The beam source 15 comprises generally an electron emissive cathode 16, a control electrode 17, and a focusing and accelerating assembly 18. The construction and operation of the beam producing source 15 is that which is well known in the art and, in the particular embodiment shown, typical operating voltages would be as follows. The cathode would be operated at ground potential while the control electrode 17 would be operated slightly negative with respect to the cathode 16. The focusing and accelerating assembly 18 may be operated at approximately 0.5 kilovolt positive with respect to the cathode.

Also disposed within the portion 12 are a series of deflection plates or electrodes 19, 20 and 2-1 which are connected to suitable sources of varying voltage to deflect the electron beam produced by the assembly 15 and cause it to be scanned over an input screen 30.

The input screen 30 is disposed at the opposite end of the envelope 10 near the faceplate 13. In the present embodiment, the input screen 30 is shown to be one which is sensitive to visible, ultraviolet or infrared radiation and includes a support member 3 1 which is of suitably transmissive material such as glass or aluminum oxide film. A layer 32 is disposed upon the surface of the support member 31 (facing the assembly 15) and is of a suitable electrically conductive material transmissive of the input radiation, for example a very thin gold or stannous oxide film. This layer 32 may also serve as the infrared absorbing layer. Disposed upon the layer 32, and in thermal contact therewith, is a layer 33 of suitable radiation responsive material such as antimony trisulphide, arsenic trisulphide or arsenic triselenide. A lead 34-, extending to the outside of the envelope 10, is provided to connect electrically the conducting layer 32 with a suitable source of potential, for example battery 60 via signal isolating resistor 61. In the present embodiment, the conducting layer 32 is operated at a potential of about volts. An alternative structure would provide that, with suitable precautions, the faceplace 13 may serve as the support in lieu of member 31 and the layer 32 would be deposited directly on the faceplate 13 as is done in the usual vidicon.

Inter-posed between the deflecting plates 21 and the input screen 30 are a plurality of cylindrically-shaped conducting member 35, 36 and 37 which collectively form a collimating electron lens system. The elements 35, 36 and 37 are provided with suitable sources of potential which may be about 0.5 kilovolt, l kilovolt and 2 kilovolts, respectively. A decelerator grid assembly 38 which may be at the same potential as the lens 37 is provided in close proximity to the input screen 30. The decelerator grid 38 serves to collect at least a portion of any secondaries emitted from the layer 33, as well as providing, between it and the layer 33, a uniform electric field in which the speed of the electron beam may be decreased without substantially changing its direction or diameter before it is incident upon the layer 33.

A second electron beam source 40 is disposed within the envelope 10 near to the deflection plates 21. This second beam source 40 consists generally of an electron emissive cathode 4'1 and a control electrode 42. The electron source 40 is of the flood beam type, that is, one which serves to direct a beam of electrons uniformly over the entire surface of the layer 33. Flood guns are well known in the art of display storage tubes and will not, therefore, be discussed in detail. In the present embodiment, the cathode 4 1 is provided with a suitable source 70 of potential in the order of 30 volts negative with respect to ground.

In the specific embodiment shown, the device is responsive to thermal radiation. Radiations from a scene are projected through a suitable lens system 29 onto the thermal sensitive target 30 and translated thereby into a distributed charge image on the layer 33. An electron beam from the electron gun is utilized to read the charge image and convert the charge image into electrical signal for utilization as an output signal. An electron flood beam from the assembly 40 is utilized to enhance the distributed charge pattern on the target layer 33 in a manner to be discussed more fully later.

As has been previously stated, the layer 3 3 is one which is sensitive to heat and as such changes its electrical conductivity in response to a change in temperature. That is, the electrical conductivity increases as the temperature of the layer increases. Though in fact it is a continuous layer, the layer 33 is viewed to be in the nature of a very large number of very small discrete elements. Thus, it is seen that each of these elements will take on an individual temperature in accordance with the thermal image on the faceplate side of the layer 33. It is essential in this type of device that each of the discrete elements has a capacitance to ground which is greater than the capacitance between the elements. In view of the difference in conductivity due to the difference in temperature of the discrete elements, and because there is a potential difference between the two surfaces of the layer 33, different values of currents will be induced in the discrete elemental areas. These currents produce a charge pattern on the surface of the layer 33 which faces the grid 38. This charge pattern will correspond to the thermal pattern of the layer 33 which, in turn, corresponds to the pattern of the incident radiation.

In normal vidicon type operation, the charge pattern on the surface of the layer 33 is scanned by means of an electron beam from the source 15 and an output signal corresponding to the value of the charge pattern on the layer 33 is derived through the conducting layer 32 and is directed to a suitable output utilization circuit 50.

In the operation of a device as has been described thus far, it is evident that certain limitations exist as to the minimum intensity of the radiation impingement upon the target which will produce a resolvable charge pattern on the surface of the layer 33. While it is theoretically true that any amount of radiation impingement upon the target 30 will produce a charge pattern thereon, as a practical matter, low input radiation levels produce only a latent image which may be obscured by minor voltage fluctuations and aberrations of the device itself. These fluctuations are random in nature and arise from the thermodynamics and statistics, and are therefore noises with an irreducible minimum. It is desirable, therefore, that some means be included within the device to develop the electrostatic latent image on the surface of the layer 33 so as to enhance the strength and contrast of the output image, and thus increase the speed and resolution with which the image may be read out.

In the present invention, such a latent image development assembly is included in the form of a second electron beam source 40 located within the envelope 10. In the preferred embodiment, this second electron beam source 40 is in the form of a flood gun which uniformly floods the exposed surface of the layer 33 with electrons. The electron flood gun 40 consists basically of a cathode 41 and a control electrode 42 which is at a certain potential more negative than the cathode 41. The gun assembly 4-0 is located approximately at the juncture of the two tubular sections Hand 12.

The manner in which the present invention serves to develop an electrostatic latent image on a target member is best explained with respect to FIG. 2. In FIG. 2 there is shown a typical secondary emission curve for a material which is capable of possessing a charge pattern as is utilized as layer 33 of the target structure 30. In FIG. 2 there is plotted as the ordinate the net current to charge of the target-free surface of layer 33 and as the abscissa the potential of the target-free surface of layer 33 with respect to the development cathode 41. In the figure, point A represents the point of first crossover or, that point at which there is no loss or gain of electrons at the target surface due to secondary emission. That is, at point A, the secondary emission ratio is unity. The portions of the curve above the abscissa represent voltages at which the secondary emission ratio is greater than unity and those portions of the curve below the abscissa represent voltages at which the secondary emission ratio is less than unity.

It is imperative in the operation of the present device that the potential difference between the front surface of the layer 33 and the development cathode 41 is such that upon an increase of this potential, there is an increase in the secondary emission. In FIG. 2, that portion of the curve which represents an increase in secondary emission with an increase in voltage lies substantially between the points B and C. The points B and C are at the extremities of the curve and are the points at which the rate of change of secondary emission changes its mathematical sign. On the portion of the curve between points B and C, the slope of the curve is negative. This slope can be interpreted as the plate resistance of the development flood beam. This view shows the plate resistance to be a negative resistance in this region of operation.

To more fully understand the operation of the device of the present invention, assume three elemental areas of the layer 33 each possess a charge thereon represented by points X, Y and Z on the curve of FIG. 2. If the layer 33 is now uniformly showered or flooded with electrons from the electron beam source 40, it is apparent that the elemental area represented by point X will have the fewest number of secondary electrons emitted and will, in fact, retain a greater number of electrons from the flooding beam than either the two other points. Thus, point X will have a net gain of electrons which tends to make the charge of point X more negative, or tends to drive it towards point B on the curve. Point Y, which is also below the point of first crossover, has a secondary emission ratio of less than unitary and, like point X, will become more negative upon being showered by electrons.

However, because point Y is closer to the point of first crossover, its retention of electrons is not as great as that of point X, and while it will tend to be driven towards point B on the curve, the rate at which this charging in the negative direction takes place is not as fast as that of point X. Point Z, on the other hand, is above first crossover and has a secondary emission ratio of greater than unity. Thus, point Z when showered by the flood gun electrons will have a net loss of electrons and hence will go more positive; that is, it will tend to go towards point C on the curve. From this, it is evident, that each charged element of the layer 33 will charge at a differential rate according to the charge which is initially on that particular element. The portions of the target which are above first crossover charge in the positive direction and those below the point of first crossover charge in the negative direction. The rate at which these elements develop is dependent upon their initial charge. That is, each elemental area charges at a diflerential rate depending upon its potential and its relation to the secondary emission curve.

If the device is to be capable of halftone reproduction, it must be time and current limited to provide that the target areas do not, upon being flooded by the second beam source 40, exceed the boundaries defined by points B and C on the curve. Points B and C are the maximum and minimum points on the secondary emission curve and are the points where mathematical sign of the rate of change of secondary emission with respect to voltage changes from positive to negative.

With respect to target areas which initially have a charge or potential corresponding to a secondary emission ratio of less than unitary, this requirement is illustrated by points R and S on the curve (FIG. 2). Assuming two elements of the layer 33 are at potentials represented by R and S and that these elements are uniformly flooded by an electron beam, it is apparent that point S will charge in the negative direction more quickly than point R. This results in point S gradually overtaking point R as they both approach point 0. Thus, all target areas which are initially below first crossover and are charged beyond point B will assume the same potential (that of O or cathode potential) and that portion of the pattern will be lost. This is exactly what occurs during reading in the vidicon.

In a similar manner, those target areas which initially have a charge or potential corresponding to a secondary emission ratio of greater than unity will, if charged beyond the defined point C, tend to assume the same potential. In any case, operation on the improper portion of the curve results in diminution rather than enhancement of the charge information on the layer 33.

Thus far, only the AC. or signal portion has been considered. If the unwritten or reference areas of the target surface are exactly at the potential of first crossover, there will be no change in the potential during development and the DC. level will remain unchanged. However, \as these unwritten areas are usually below or above the potential of first crossover, the DC. level will be lowered or raised in proportion to the net deposition or removal of electrons on them during development. Circuit elements should be incorporated to correct or account for this D.C. level shift as required in each particular application, usually by proper adjustment of the reading gun cathode 16 potential with respect to layer 32 potential.

The time constant of the layer 33, that is, the length of time it will hold a charge pattern will, in part, determine the manner in which the flood gun 40 is used. If the target is one having a long time constant, as in a storage tube, the flood g-un may be used sequentially with the Writing and reading phases of the tube, that is, the charge pattern may be written on, the flood gun utilized to enhance the charge pattern, and then the charge pattern may be read out. If the pattern is one having a small time constant, it may be desirable to utilize the flooding feature simultaneously with the writing-on portion of the cycle.

In the case of tubes having resistive storage targets, for example, vidicon, the analysis of the development action is better explained by reference to the transfer characteristic of the development electron beam and a load line which represents the storage target resistance. The action is exactly the same whether the target voltage supply is above or below first crossover, though the operating region of the secondary emissive curve is different. Reference to 'FIG. 3 shows that the load lines representing the target resistance determine the voltage at the target surface for which the dark current is just equal to the net current from the development flood beam. In FIG. 3, R represents the dark resistance of the layer 33, R represents the highlight resistance of layer 8-3 due to photoconduction or other causes, and V and V represents target supply voltages. Variation of the target resistance, for example by photoconduction in the case of the vidicon, varies the slope of the load line and the target surface potential as the intersection shifts. This latter shift is accentuated by the negative slope of the characteristic curve. This accentuation is increased as the difference between the target voltage and the first crossover voltage decreases.

This graphical analysis can "be better understood by reference to the equivalent circuit (FIG. 4). Since the plate resistance of the flood beam action at the retina surface is non-linear (in fact, a negative resistance in the chosen separating region), it is common practice to use this graphical solution as in analysis of similar circuits.

Note that the intersection of the load line with the characteristic curve gives a quiescent operating point for a particular level of illumination. The corresponding reading current is also a fixed value. These are not transient conditions as are usual in the vidicon and image orthicon as normally operated without an axiliary beam. A somewhat larger reading output and thus still more sensitive operation can be obtained by allowing a large reading beam current to discharge the retina element capacitance in a transient manner, but the equilibrium voltage of the retina surface is still the quiescent point determined by the intersection of the curves as described above.

The introduction of the flood beam adds a negative resistance in parallel to the resistor-capacitor network of the retina element to increase its time constant. This allows storage and signal integration on the retina element in proportion to this increase in time constant. This permits the increased detectivi-ty with increased frame time.

This increase in retina time constant has an additional advantage. It can be used to enable the use of retina materials which have a high inherent responsivity, but are too highly conductive to perm-it signal integration over a practical frame time. These materials are practically useless since the response of a complete camera tube utilizing them is too low, but the addition of the flood beam lengthens their time constant and permits them to be used so that their inherent greater responsivities augment the gain obtained by the electrostatic latent image development alone.

The reading beam cathode must be adjusted with respect to the quiescent potentials of the dark areas and the highlights so that these potentials, and those between them of intermediate illumination, fall on the positive slope of the reading characteristic. There are several ways well known to the art to accomplish this. If it is done as in a vidicon, near the reading gun cathode potential, as shown in FIG. 5, the reading erases some of the information so that new information is integrated on the retina surface during each reading frame.

While thus far .the invention has been described with respect to a vidi-con type of device, it is readily apparent that the same principles would apply in other types of devices, for example, those using return beam readout such as is present in an image orthicon. The necessary changes to modify the above description with respect to a return beam type tube, are readily apparent to those skilled in the art, the essential portion of the invention being that there is a target present capable of possessing a charge pattern thereon and that this charge pattern is developed by means of a second electron source at the proper potential so that enhancement of the charge pattern is achieved.

While there has been shown and described what is at present considered to be the preferred embodiment of the invention, modifications thereto will readily occur to those skilled in the art. 'For example, While in the preferred embodiments the second electron beam source is of the flood igun type, it is readily apparent that a beam which is more restricted in area and which scans the storage surface, could be utilized with equal success.

It is not desired, therefore, that the invention be limited to the specific arrangement shown and described, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

We claim as our invention:

1. An electron discharge device comprising an envelope, target means within said envelope for the establishment of a voltage pattern corresponding to an information pattern continuously incident thereon, means including an electron source for continuously and uniformly flooding said target with electrons, and potential means connected between said target and said electron source, said potential means providing that electrons from said electron source are continuously incident upon said target with an energy corresponding only to that portion of the secondary emission curve of the target material in which the rate of secondary emission increases with an increase in potential to provide an enhanced halftone voltage pattern of said voltage pattern.

2. An electron discharge device comprising an evacuated envelope, target means within said envelope capable of possessing a voltage pattern thereon, means for continuously exciting said target to establish said voltage pattern, electron beam producing means for the projection of electrons onto said target, potential means connected between said target and said electron beam producing means, said potential means providing to provide that electrons from said source are incident upon said target with energy corresponding only to that portion of the target materials secondary emission curve in which the rate of secondary emission rises with an increase in said potential to provide an enhanced halftone voltage pattern of said voltage pattern and means for deriving an output signal from said target.

3. An electron discharge device comprising an envelope, target means within said envelope, means for the excitation of said target to establish a voltage pattern thereon, electron beam producing means for the continuous projection of electrons onto said target, electrical potential means between said target and said beam producing means, said potential means providing a continuous supply of electrons such that a rise in said potential effects an increase in secondary electron emission from said target and permit halftone reproduction, and means including an additional electron beam producing means for deriving an output signal from said target.

4. An electron discharge device comprising an envelope, target means within said envelope for the establishment of a charge pattern corresponding to an information pattern incident thereon, means for directing an electron beam onto said target to write said charge image, means including an electron source for uniformly flooding said target with electrons, and potential means between said target and said electron source, said potential means providing electrons from said electron source that are incident upon said target with an energy corresponding only to that portion of the secondary emission curve of the target material in which the rate of secondary emission increases with an increase in potential, said electron flooding means operable to enhance the charge image and reduce the writing time.

5. An electron discharge device comprising an evacuated envelope, a resistive target means within said envelope for establishing a voltage pattern thereon, means for the excitation of said target to modify the resistance of said target and establish said voltage pattern, means including a source of electrons to uniformly flood said target with electrons, potential means between said target and said electron source, said potential means providing electrons from said source incident upon said target with energy corresponding to that portion of the target materials secondary emission curve in which the rate of change secondary emission is positive with respect to an increase in said potential, said electron flooding means operable to provide enhancement of said voltage pattern, and means for deriving an output signal from said target.

6. An electron discharge device comprising an envelope, target means within said envelope, said target means exhibiting the property of producing a resistance image corresponding to a radiation image impingent thereon, electron beam producing means for uniformly flooding said target with electrons While said target is exposed to said impingent radiation, an electrical potential means between said target and said beam producing means, said potential means providing electrons of an energy that a rise in said energy effects only an increase in secondary emission from said target, and means including a second electron beam producing means for deriving an output sipnal from said target.

7. A method of enhancing an electrostatic voltage pattern on a radiation sensitive resistive target comprising the steps of providing a target excitable to produce a voltage pattern thereon, exciting said target to change the conductivity and efiect said voltage pattern, enhancing the strength of said voltage pattern by the impingement of an electron beam thereon, said electron beam continuously impingent upon said target with a potential with respect to said target to providethat the rate of change of secondary electron emission from said target is positive values of said voltage pattern through said secondary electron beam serving to change differentially the relative with respect to an increase in said potential, said electron emission.

8. The method of producing an enhanced halftone in an image device comprising the steps of providing a target within the envelope of said image device, directing writing information onto said target to establish a voltage pattern on said target, directing an enhancement electron beam onto said target to enhance the strength of said voltage pattern, said enhancement electron beam having an energy with respect to said target to provide that the rate of secondary emission from said target in response to bombardment increases with an increase of electron energy and directing a reading electron beam onto said target to derive a signal representative of the enhanced voltage pattern on said target.

References Cited by the Examiner UNITED STATES PATENTS 2,754,449 7/1956 Farnsworth 3l3--7l X 2,788,466 4/1957 Hansen 315-12 2,839,679 6/1958 Harris 31513 X 2,843,799 7/1958 Hook et al 315l3 X 3,086,139 4/1963 Lehrer 31371 X DAVID G. REDINBAUGH, Primary Examiner.

ARTHUR GAUSS, Examiner.

C. O. GARDNER, J. E. BECK, Assistant Examiners. 

2. AN ELCTRON DISCHARGE DEIVCE COMPRISING AN EVACUATED ENVELOPE, TARGET MEANS WITHIN SAID ENVELOPE CAPABLE OF POSSESSING A VOLTAGE PATTERN THEREON, MEANS FOR CONTINUOUSLY EXCITING SAID TARGET TO ESTABLISH SAID VOLTAGE PATTERN, ELECTRON BEAM PRODUCING MEANS FOR THE PROJECTION OF ELECTRONS ONTO SAID TARGET, POTENTIAL MEANS CONNECTED BETWEEN SAID TARGET AND SAID ELECTRON BEAM PRODUCING MEANS, SAID POTENTIAL MEANS PROVIDING TO PROVIDE THAT ELECTRONS FROM SAID SOURCE ARE INCIDENT UPON SAID TARGET WITH ENERGY CORRESPONDING ONLY TO THAT PORTION OF THE TARGET MATERIAL''S SECONDARY EMISSION CURVE IN WHICH THAT RATE OF SECONDARY EMISSION RISES WITH AN INCREASE IN SAID POTENTIAL TO PROVIDE AN ENHANCED HALFTONE VOLTAGE PATTERN OF SAID VOLTAGE PATTERN AND MEANS FOR DERIVING AN OUTPUT SIGNAL FROM SAID TARGET. 